High reverse breakdown voltage and low on-resistance are required for high-current switching devices. Field effect transistors (FETs) that use a group III nitride-based semiconductor are excellent in terms of, for example, high breakdown voltage and high-temperature operation because of their wide band gap. Vertical transistors that use a GaN-based semiconductor have been particularly receiving attention as transistors for controlling high power. For example, PTL 1 proposes a vertical GaN-based FET whose mobility is increased and whose on-resistance is decreased by forming an opening in a GaN-based semiconductor and forming a regrown layer including a channel of two-dimensional electron gas (2DEG) in a wall surface of the opening. In this vertical GaN-based FET, a structure including a p-type GaN barrier layer and the like is proposed in order to improve the breakdown voltage characteristics and pinch-off characteristics.